Synthesis of superconducting nb-sn

ABSTRACT

A method comprising:
         electrodepositing a film comprising a Nb—Sn material onto a copper substrate surface from an electrolyte bath comprising (a) SnCl 2 , (b) NbCl 5 , and (c) (i) 1-Ethyl-3-methylimidazolium chloride (EMIC), (ii) 1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture thereof.

This application claims the benefit of U.S. Provisional Application No. 62/190,199, filed Jul. 8, 2015, which is incorporated herein by reference.

BACKGROUND

The Nb₃Sn intermetallic compound is a high performing superconductive material which finds wide application in Nuclear Resonance Magnetic devices, high field lab magnets, but also fusion and accelerator magnets.

Manufacturing a film-based superconducting radio frequency (SRF) structure remains a “holy grail” for accelerator physicists. The main reason for aiming at thin film surface coating is that one can, in principle, improve performance and save on costs, since with penetration depths of 40 to 100 nm only ˜1 μm in thickness would be needed for the superconducting film. Within the new EUCARD Program, which was started in Europe last year, the thin film activity is distributed between four labs, including Helmholtz-Zentrum Berlin fur Materialien und Energie (HZB), CEA Saclay, INP Grenoble and the European Organization for Nuclear Research (CERN). The most successful film-based Cu cavities are those made at CERN, which have reached accelerating gradients in excess of 20 MV/m. More recently promising results for Nb/Cu cavities were obtained with High Impulse Power Magnetron Sputtering (HIPIMS) as compared to the standard dc Magnetron Sputtering.

On the other hand, the manufacturing of superconductive Nb₃Sn films is still in its infancy, and film-based SRF cavities have not yet equaled the performance of conventional bulk niobium cavities. There are two main problems with film-based cavities. The first is the presence of defects within one or two penetration depths from the surface. In fact H_(c1) (at defects) is expected to be much lower than H_(c1) (bulk). Defects are particularly numerous in films produced at low temperatures. Oxygen and hydrogen will be trapped at defects, and could have significant negative impact at high gradients. The second issue concerns the grain size for the Nb/Cu films, which is about 100 nm, i e 10,000 times smaller than the grain size of conventional SRF cavities. Grain boundary diffusion and trapping of oxygen and hydrogen are much faster than impurity diffusion in bulk Nb. It is known that grain boundary scattering is the main reason for a low residual resistivity ratio (RRR) in films compared to bulk niobium of similar purity. Yet the surface resistance of film-based SRF cavities at low currents is at least as low as for the bulk materials. This means that the quality factor Q₀ of film based cavities could be at least as good as for high quality bulk Nb cavities. However, the present performance of film-based cavities at high acceleration gradients is at lower Q values.

Kolosov et al. deposited Nb and Nb3Sn from high-temperature molten salt solutions containing LiF—NaF—KF, NaCl—KCI and LiCI—KCI (e.g., Kolosov, V. N. and Matychenko, E. S., Evaluation of High Frequency Superconductivity of Niobium Coatings Prepared by Electrodeposition Process in Molten Salts, in Refractory Metals in Molten Salts, Dordrecht: Kluwer, 1998, pp. 231-238). However due to the aggressive condition of deposition (corrosivity and temperature from 400 to 1000 K) only few substrates can be utilized. The most promising alternative to those high temperature electrolytes are low temperature ionic liquids. Early attempts to deposit Nb—Sn alloys from ionic liquids were performed by Koura et al. (Ito H. Koura N., Ling G. Electrodeposition of Nb—Sn alloy from ambient temperature molten salt electrolytes, Hyoumen Gijutsu, 46(12), 1162-1166(1995). They recorded cyclic voltammetries for 1-butylpyridinium chloride (BPC)-NbCl₅ and BPC-NbCl₅—SnCl₂ solutions. Moreover they investigated the deposition of Nb—Sn alloys from 53.8% SnCl₂—7.7% NbCl₅—38.5% BPC solution at 130° C., at 5 mA/cm², using a copper cathode and a tin anode. The resulting Nb—Sn film contained about 14.8 wt % Nb. The deposition from 7.7% SnCl₂—15.4% NbCl₅—76.9% BPC bath at 40 mA/cm² and 130° C. resulted in a Nb composition of 27.9 wt %. In a second paper the same group evaluated the effects of a pulse electrolysis on a 28.6% SnCl₂—14.3% NbCl₅—57.1% BPC bath. Ito H. Koura N., Ling G. Electrodeposition of Nb—Sn alloy from ambient temperature molten salt electrolytes by pulse electrolysis, Hyoumen Gijutsu, 48 (4), 454-459 (1997). They concluded that the niobium content in the Nb—Sn electrodeposit was affected by pulse period, current density and duty ratio: decreasing the duty ratio and increasing the current density increased the deposited Nb content. They deposited a Nb—Sn alloy containing 44.3 wt % Nb, at current density of 60 mA/cm², t=50 ms and a duty ratio of 0.2. They also deposited a 41.3 wt % Nb alloy from a 7.7% SnCl₂—15.4% NbCl₅—76.9% BPC bath at 60 mA/cm², t=10 ms and a duty ratio of 0.2. However, none of the papers gave evidence of the presence of a superconductive phase. The same group also reported that the electrodeposition of a Nb—Sn alloy can be done from a SnCl₂—NbCl₅ solutions in 1-ethyl-3-methylimidazolium chloride (EMIC). N. Koura, T Umebayashi, Y Idemoto and Gouping Ling, Electrodeposition of Nb—Sn Alloy from SnCl₂—NbCl₅-EMIC Ambient Temperature Molten Salts, Electrochemistry, 67(6), 689(1999). Electrodeposition carried out in constant current resulted in a very low niobium content in the alloy. On the contrary, pulse plating from an acidic melt with a 2.8% SnCl₂—68.6% NbCl₅—28.6% EMIC with a pulse period of 10 ms, duty ratio of 0.2 at 160° C., increased the Nb concentration to 69.1 wt %. XRD analysis and resistivity tests demonstrated that a superconductive Nb₃Sn phase could be obtained. However, the same authors declared that the reproducibility of the process was not acceptable.

To overcome the limits of Lewis acidic ionic liquids, more recently Koichi et al also tested a Lewis basic melt consisting in 4.4% SnCl₂—95.6% EMIC. Koichi Ui, Sakai H, Takeuchi K., Ling G., Koura N., Electrodeposition of Nb₃Sn Alloy Film from Lewis basic SnCl₂—NbCl₅-EMIC melt, Electrochemistry, 77 (9) 798-800 (2009). Cyclic voltammetries were carried out at 130° C. and 10 mV/s and reduction and oxidation waves were clearly observable in the potential range (vs Al(III)/Al) from −1.07 V to −1.30 V and from 1.05 V to −0.75 V respectively. The cyclic voltammetry of the 11.9% NbCl₅—88.1% EMIC basic melt showed reduction waves from 0.21 V and −0.77 V vs Al(III)/Al. A similar profile was obtained by Sun et al. from a 49.0% AlCl₃—51.0% EMIC containing NbCl₅. I-Wen Sun and Charles L. Hussey, Electrochemistry of Niobium Chloride and Oxide Chloride Complexes in the Basic Aluminum Chloride-1-Methyl-3-ethylimidazoliumChloride Room-Temperature Ionic Liquid, Inorg. Chem. 1989, 28, 2731-2737. Koichi et al. performed cyclic voltammetry of the 19.2% NbCl₅—10.0% SnCl₂—70.8% EMIC Lewis basic melt and observed two reduction waves at a potential lower than −0.5 V vs Al(III)/Al. The reduction wave of niobium ionic species was identified as the one at −0.4 V vs Al(III)/Al, while the second peak at −0.8 V vs Al(III)/Al was attributed to the reduction of Sn, indicating that the co-deposition reaction of Nb and Sn might occur because both reduction potentials were very close to each other. The electrodeposition from this melt was attempted using a constant current pulse method at 0.1 A/cm² with a pulse period of 10 ms, duty ratio of 0.20, and electricity of 5 C/cm² at 130° C. The electrodeposition cell consisted in a copper working electrode, a Sn counter electrode and an Al(III)/Al quasi reference electrode. The XRD pattern of the electrodeposit revealed the presence of Nb₃Sn along with metallic Sn and Cu₁₀Sn₃ alloy, which was attributed to the solid phase interdiffusion between the copper substrate and Sn atoms due to the high bath temperature. However, the presence of Sn and Cu—Sn phases could also be attributed to the dissolution of the Sn anode during the electrodeposition process. While anode dissolution might be advantageous for some metal deposition, in this case the choice of Sn appears to be problematic. In fact, Sn already shows good solubility in the solvents and it is more prone to reduce at the cathode than it is Nb. Furthermore, the aluminum wire was also soluble in the same electrolyte, making it an unsuitable material as quasi-reference electrode. Finally, no indication of the total thickness of the film was given, which is a property of great interest to make the approach eligible for applications and further development to the industrial scale.

In addition, the intermetallic compound Nb₃Sn is a type II superconductor having a well-defined stoichiometry and the A15 crystal structure. It has a critical temperature T_(c0) of up to 18.3 K and an upper critical magnetic field B_(c20) of up to 30 T. As a comparison, the ductile alloy NbTi has a T_(c0) of 9.3K and a B_(c20) of 15 T, which make NbTi adequate only up to operational magnetic fields of 8 to 9 T, as in the case of the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN, Switzerland), whose NbTi magnets operate in superfluid helium at 1.9K to bend and collide proton beams and eventually reach an energy of 14 TeV in the center of mass. Superconducting materials have found a wide range of applications in science and society. Their unique properties and exquisite sensitivity have been exploited in many science disciplines. Superconductivity is used in detectors for dark matter, for the cosmic microwave background radiation and for national security purposes. Superconducting magnets and radio frequency (SRF) structures are at the heart of most particle accelerators for fundamental science, as well as accelerators for medical isotope production and ion therapy treatment. Superconductivity is also being explored for use in biosensors and quantum computing. Thanks to Nb₃Sn's stronger superconducting properties, it enables magnets above 10 T, which for instance is a larger field than any existing in present NbTi particle accelerators. Nb₃Sn is also the superconductor of choice for high field magnets to be used for plasma confinement in fusion reactors. The International Thermonuclear Fusion Research and Engineering project (ITER, France) uses a Central Solenoid of 13.5 T. But perhaps the most extensive use of Nb₃Sn is for Nuclear Magnetic Resonance (NMR) spectrometers, which have become a key analysis tool in modern biomedicine, chemistry and materials science. These systems use fields up to 23.5 T, which correspond to a Larmor frequency of 1000 MHz.

Some of the challenges are that Nb₃Sn requires high-temperature processing, which makes it brittle, and its critical current is strain sensitive, i.e. high strain on the sample may reduce or totally destroy its superconductivity. In the last decades, several manufacturing processes have been developed to produce superconductive Nb₃Sn wires, including the bronze route, the powder-in-tube method, and internal tin, which includes as variants the modified jelly roll and the Restacked Rod processes (RRP®). In the last 15 years, Fermi National Accelerator Laboratory (Fermilab, US) has used these wires and developed superconducting cables to perform Nb₃Sn research for high field accelerator magnets. The Fermilab High-Field Magnet Group built the first reproducible series in the world of single-aperture 10 to 12 T accelerator-quality dipoles made of Nb₃Sn, establishing a strong foundation for the LHC luminosity upgrade at CERN. More recently, the first successful twin-aperture accelerator magnet made of Nb₃Sn and developed and fabricated at Fermilab reached its design field of 11.5 Tesla at 1.9K.

SUMMARY

Disclosed herein is a method comprising:

electrodepositing a film comprising a Nb—Sn material onto a copper substrate surface from an electrolyte bath comprising (a) SnCl₂, (b) NbCl₅, and (c) (i) 1-Ethyl-3-methylimidazolium chloride (EMIC), (ii) 1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture thereof.

Also disclosed herein is a method comprising:

electrodepositing a seed copper layer onto a surface of a Nb substrate;

electrodepositing a tin layer onto the seed copper layer;

electrodepositing a copper barrier layer onto the tin layer to form an intermediate construct; and

heating the intermediate construct to form a Nb₃Sn coating.

The foregoing will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C. CV curves of pure Emim-Cl compared to: FIG. 1A—10% SnCl2 90% Emim-Cl; FIG. 1B—25% NbCl5 75% Emim-Cl; FIG. 1C—75% Emim-Cl—21% NbCl₅—4% SnCl₂.

FIGS. 2A-2C2. FIG. 2A: polarization curves at constant current in the range 10-50 mA/cm²; FIG. 2B and FIG. 2C: SEM micrograph and XRD pattern of a Nb—Sn coating deposited from Bath 1 at 40 mA=cm2 and 130° C.

FIGS. 3A-3C. CV curves of pure Bmim-Cl compared to: FIG. 3A—10% SnCl₂ 90% Bmim-Cl; FIG. 3B—25% NbCl5 75% Bmim-Cl; FIG. 3C—75% Bmim-Cl—21% NbCl₅—4% SnCl₂.

FIG. 4. XRD patterns of Nb—Sn films electrodeposited onto Cu substrates from from Bath 2 at 40 mA/cm2: 28at. % Nb, film thickness 750 nm.

FIGS. 5A-5D. SEM micrographs (5A and 5B) and XRD patterns (FIGS. 5C and 5D) of Nb—Sn films electrodeposited onto Cu substrates from Bath 2 at 400 mA/cm²: 17at. % Nb, film thickness 200 nm.

FIGS. 6A-6D. Nb—Sn film on Cu substrate, from Bath 3 at 130° C. and 40 mA/cm² for 4 h 30 min: FIG. 6A—XRd pattern; FIG. 6B—AFM micrograph; FIG. 6C—SEM micrograph; FIG. 6D—In depth concentration profile by EDX analysis.

FIGS. 7A-7C. Nb—Sn film on Cu substrate, from Bath 4 at 130° C. and 400 mA/cm² for 25 min: FIG. 7A—SEM micrograph; FIG. 7B—GDOES in-depth profile; FIG. 7C—XRD pattern.

FIG. 8. Thickness of Sn coatings on copper substrates (a) and on Nb substrates (b) as a function of deposition time.

FIGS. 9A-9D. Copper seed layer onto Nb substrate (FIG. 9A), Sn coating onto Nb/Cu substrates (FIG. 9B), XRD pattern of a Nb/Cu/Sn sample (FIG. 9C) and final sample design for superconductivity tests (FIG. 9D).

FIG. 10. Thickness of the Nb-Sn phase as a function of the duration of the heat treatment, compared to data reported in literature.

FIG. 11. Temperature vs. time profile of heat treatments performed on Nb/Cu/Sn/Cu samples.

FIGS. 12A-12B. XRD pattern (FIG. 12A) and GDOES analysis (FIG. 12B) of sample consisting of Nb/Sn(10 μm)/Cu(10 μm) after thermal treatment (sample 53).

FIGS. 13A-13B. XRD pattern (FIG. 13A) and GDOES analysis (FIG. 13B) of a Nb/Sn(15 μm)/Cu(15 μm) sample after thermal treatment (sample 56).

FIGS. 14A-14B. GDOES analysis (FIG. 14A) and XRD pattern (FIG. 14B) of a Nb/Sn(20 μm)/Cu(15 μm) sample after thermal treatment (sample 59).

FIGS. 15A-15B. Critical current density of Nb—Sn thin films as a function of magnetic field in the perpendicular (FIG. 15A) and parallel (FIG. 15B) orientations.

DETAILED DESCRIPTION

As used herein, “Nb—Sn” refers to a material that comprises Nb and Sn (in preferred embodiments the material contains only Nb and Sn, and substantially no other element or contaminant). The Nb—Sn material may be present in various stoichiometric or nonstoichiometric ratios (e.g., Nb₃Sn and/or NbSn₂) and phases.

Electrodepositing Nb—Sn Coatings on Copper Substrates

Disclosed herein are method for electrodepositing Nb—Sn coatings on copper substrates from SnCl₂—NbCl₅-EMIC and SnCl₂-(NbCl₅)-BMIC ionic liquids. In certain embodiments, the copper substrate is substantially pure copper. Two different approaches were followed: Nb cations were provided to the electrolyte either by direct addition of NbCl₅ salt or by electrochemical dissolution from the Nb anode. Cyclic voltammetric curves were recorded to investigate the electrochemical behavior of the electrolyte. Reduction waves of Nb and Sn ionic species were clearly identified. Electrodeposition was performed in constant current mode at 40 and 400 mA/cm² and at a temperature of 130° C. Phase structure and texture, composition and morphology were determined by X-ray diffraction (XRD), X-Ray Fluorescence (XRF), glow discharge optical emission spectrometry (GDOES), Laser profilometry, Scanning electron microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis. The maximum niobium content in electrodeposited Nb-Sn films was 63 at %. Film thickness was in the range from 200 to 4000 nm and average surface roughness in the range of 0.230÷2.113 μm depending on the operating parameters. The electrodeposited coatings showed a cubic Nb₃Sn phase with (211) preferred orientation and an orthorhombic NbSn₂ phase. These results demonstrate depositing thin superconducting layers on copper surfaces with a relatively simple and inexpensive method. In certain embodiments, the minimal layer thickness for SRF applications is 1 micron, more particularly 1 to 5 microns.

The electrochemical methods disclosed herein do not require high temperature processing. Use of SnCl₂ and NbCl₅ and the ionic liquids overcomes the technological difficulty of using large area cathodes in controlled environments, and to control thickness uniformity. The main feature of electrochemical deposition techniques is that they allow molecule formation beyond the standard metallurgical phase diagrams. For instance, in the technique herein described, the Nb₃Sn film is directly produced in molecular form and at a temperature of 130° C. to 150° C., which is much lower than in any known state-of-the-art method, which is by Nb—Sn solid diffusion at temperatures greater than 650° C. Also, electrochemical deposition techniques have the advantage that they can be successfully performed on complex 3D shapes having high aspect ratio, which in the case of Nb₃Sn is impossible to obtain with the classical metallurgical techniques due to the intrinsic brittleness of Nb₃Sn. Furthermore, in principle electrodeposition techniques enable synthesizing of Nb₃Sn coatings in only one step.

Disclosed herein is the electrodeposition of Nb—Sn thin films from a bath containing SnCl₂, NbCl₅ and 1-Ethyl-3-methylimidazolium chloride (EMIC) and from a novel Lewis basic ionic liquid consisting in SnCl₂, NbCl₅ and 1-Butyl-3-methylimidazolium chloride (BMIC) (both belonging to the so-called first generation ionic liquids). 1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium ions have good viscosity and high conductivity (see Table 1). The synthesis of BMIC is an easier process than the synthesis of EMIC, which requires a pressurized reaction vessel, in fact BMIC is less expensive than EMIC. Ionic liquids containing BMIM tetrafluoroborates were successfully used to electrodeposit a variety of metals, such as Zn, Fe and Mg, BMIM hexafluorophospates were used to electrodeposit Ge, Lewis acid BMIC/chloroaluminate ionic liquids were used for the electrodeposition of Al alloys, while Lewis basic BMIC/chloroaluminate ionic liquids were used for the electrodeposition of Pd.

TABLE 1 Melting point (Tm), thermal decomposition point (Td), ionic conductivity (σ), viscosity (η) and diffusion coefficient of EMIC, BMIC, EMIM+ and BMIM+ [E]. EMIC BMIC EMIM+ BMIM+ Water Tm (° C.)  89 65 Td (° C.) 285 250 ρ (g cm⁻³) 1.256 (30° C.) σ (mS cm⁻¹) 6.5 11 4.6 (30° C.) (30° C.) (30° C.) η (cP) 47 27 40 0.89 (30° C.) (30° C.) (30° C.) (25° C.) D/10-11 (m² s⁻¹) 6.2 3.4 (30° C.) (30° C.)

Experimental Setup

The experiments were carried out using two ionic liquids based on either EMIC (>98%, Sigma-Aldrich) or BMIC (>98%, Sigma-Aldrich). Anhydrous stannous chloride (SnCl₂, 98%, Sigma-Aldrich) and anhydrous niobium chloride (NbCl₅, 99%, Sigma-Aldrich) were added to the ionic liquids to obtain the Lewis basic melts. The chemicals were mixed in a glove box (argon or nitrogen atmosphere) by magnetic stirring at 100° C. for 24 hours. Subsequent electrochemical measurements and deposition experiments were done in an electrochemical cell sealed to the ambient atmosphere. In the following, % stands for mol. %. The SnCl₂/EMIC mixtures were prepared mixing EMIC and SnCl₂ and subsequently heating at 100° C. with a light stirring. Heating the mixture is required, since both the chemicals are solid at room temperature. At 70° C. EMIC melts, and SnCl₂ easily dissolves into it by forming a transparent greenish solution. Cyclic voltammetries (CVs) were performed at 100° C. and 10 mV/s using a three-electrode cell configuration. Cu sheets (99.95%, thickness 700 μm) were used as cathode. The exposed area into electrolyte was 1-2 cm². The counter electrode was a Sn sheet. A Pt wire was employed as a reference electrode Unlike EMIC-SnCl₂, the solution formed with niobium salt is liquid at room temperature at concentrations ranging from 4% to more than 30% NbCl₅. Cyclic voltammetry was performed on a 25% NbCl₅—75% EMIC solution at 100° C. and 10 mV/s in a three-electrode cell similar to the previous one with niobium instead of tin as counter electrode.

The SnCl₂—NbCl₅-EMIC solution was obtained by adding NbCl₅ to an EMIC-SnCl₂ solution. The addition of niobium salt greatly increased the melting point of the solution, which was solid at ambient temperature and assumed a dark brown color. Electrolytes with higher niobium salts concentrations were not tested, since it was observed that they are solid even at temperatures up to 150° C. A CV was performed on a 75% EMIC-21% NbCl₅—4% SnCl₂ at 130° C. by using niobium metal as counter electrode. Platinum was chosen as reference electrode for our experiments, while aluminum or tin were used by Koura and Koichi. Platinum resulted to be stable in ionic liquids solutions, while aluminum and tin reacted when in contact with the electrolyte.

Electrodeposition from EMIC based electrolytes was carried out from a 85% EMIC-10% NbCl₅—5% SnCl₂ (in the following called Bath 1) solution at 120° C. using a two electrodes cell configuration. Cyclic voltammetries, which are electrochemical measurements, were done with a three electrode system. Electrochemical deposition experiments were done using a two electrode cell. The cathode was pure copper, the anode was pure tin or pure niobium. Galvanostatic mode was tested, but potentiostatic and pulsed modes could also be used. Cu sheets (99.95%, thickness 700 μm) were used as substrates. The exposed area into electrolyte was 1-2 cm². The anode was a Nb sheet. Experiments were carried out at a constant current density of 40 mA/cm² applied for 240 s.

A similar procedure was followed for the investigation on BMIC solutions. The CV curves were recorded on 10% SnCl₂—90% BMIC electrolyte, which was obtained by mixing the chemicals in a glove box by magnetic stirring at 100° C. for 24 hours. The as prepared solution was transparent and uncolored, with the advantage of a lower viscosity compared to the EMIC-based solutions. The 75% BMIC-25% NbCl₅ eutectic was prepared by adding NbCl₅ to BMIC at 100° C. under magnetic stirring in glove-box. The solution showed a light brown color, lower viscosity if compared to EMIC and it was liquid at ambient temperature. The SnCl₂—NbCl₅-BMIC eutectic was obtained by adding NbCl₅ to an EMIC-SnCl₂ solution. The CV curves were recorded using a solution of composition 75% BMIC-21% NbCl₅—4% SnCl₂.

The electrodeposition from BMIC-based electrolytes was carried out from a 85% BMIC-10% NbCl₅—5% SnCl₂ solution (in the following called Bath 2) in galvanostatic mode at current densities of 40 and 400 mA/cm² for times from 600 s to 1800 s at 130° C. The deposition of the Bn—Sn alloy was done on copper substrates, the exposed area being ˜1-2 cm². The counter electrode was a Nb sheet. It has to be considered that the addition of the niobium chloride greatly increased the melting point of the solution, which was solid at ambient temperature and assumed a color ranging from orange (10% NbCl₅) to dark brown (25% NbCl₅). In the following, this approach will also be referred to as “approach 1”. Alternatively, Nb was added to the electrolyte by electrolytic dissolution of the Nb anode during the electrochemical polarization. The electrolyte composition was 90% BMIC-10% SnCl₂ (Bath 3). In the following, this approach will also be referred to as “approach 2”. Despite the lower melting temperature (the commercial BMIC in particular is liquid at room temperature while EMIC is solid), electrodeposition tests were carried out at T>100° C. In fact, ¹³C and ³⁵Cl NMR spectra demonstrated that by increasing the operating temperature the geometry of the coordination complex changes and a more dissociated structure is favored, which in turn has beneficial effects on metal deposition.

Karl-Fischer analysis was performed to determine the water content in the as-prepared electrolyte using a Mettler Toledo titrator (Model DL31). The water content was found to be about 0.44 wt. % for the as prepared electrolyte containing 85% BMIC-10% NbCl₅—5% SnCl₂.

For electrochemical measurements, a potentiostat/galvanostat (Solartron Analytical ModuLab ECS) was used. The surface morphology was investigated by means of Scanning Electron Microscopy (SEM-Zeiss® EVO 50) equipped with LaB6 source, operated at 20 kV accelerating voltage. Phase structure and texture of the Nb—Sn coatings were assessed by acquiring X-ray diffraction (XRD) patterns with Cu Kα radiation (λ=1.5405 Å) and a powder goniometer (Philips PW-1830) in the 2θ angular range of 10-90°. XRD patterns were explained by means of powder diffraction references. Grain size was estimated by calculating the crystal coherence extensions according to Scherrer equation. Surface roughness was measured by generating 2D profiles using a UBM Mikrofocus® laser profilometer (UBM Messtechnik GmbH). Film thickness was measured by X-ray fluorescence using a Fischerscope-XAN®-FD BC instrument. Glow discharge optical emission spectrometry (GD-OES) depth profiling analysis was performed with a Spectruma GDA750 analyser using argon ions for sputtering with a beam spot size of 2.5 mm. Film composition was also assessed by EDX analysis.

Results and Discussion

The cathodic behavior of the ionic liquid solution was investigated by CV with the objective to define the potential range for alloy deposition.

FIG. 1 shows CV curves over the potential range from −0.5 to −3.7 V corresponding to the pure EMIC (a), EMIC with addition of 10% SnCl₂ (a), EMIC with addition of 25% NbCl₅ (b), and EMIC containing 5% SnCl₂ and 21% NbCl₅ (c). In the case of the base electrolyte, a low current density (c.d.) was observed over the potential range from OCP to −2.0 V, significantly increasing towards the lower limit of the scanning range, possibly due to the reduction of ionic liquid itself or to the electrochemical decomposition of the water contained in the electrolyte. In fact, a main shortcoming of the first generation ionic liquids (EMIC and BMIC) is that the organic halides can easily contain more than 1000 ppm of water (up to 10 000 ppm of water), this water possibly reacting even with metal chlorides. However pure EMIC shows a relatively wide potential window. By adding 10% SnCl₂ to pure EMIC (FIG. 1-a), a broad cathodic peak appears at about −0.75 V, with a peak of about 1.6 mA/cm². Since the melt is basic, SnCl₃ ⁻ is the main anionic species in the melt, and Sn deposits or dissolves according to the following equation:

SnCl₃ ⁻+2e→Sn+3Cl⁻  Eq. 1

At higher cathodic potential, the c.d. approaches the value observed for the base electrolyte, shifting water or EMIC decomposition at −0.83 V. Galvanostatic deposition experiments carried out at 80° C. and 10 mA/cm² for 600 s resulted in a bright and uniform metallic Sn film (not shown).

By adding 25% NbCl₅ to pure EMIC (FIG. 1-b), two reduction peaks were observed at −0.86 V and c.d. 4.0 mA/cm², and −1.83 V vs Pt and c.d. of 5.6 mA/cm². A similar profile was obtained by Sun et al. from a 49.0% AlCl₃—51.0% EMIC containing NbCl₅. Sun et al. attributed the two reduction peaks to reactions involving the niobium anionic species according to the subsequent equations:

NbCl₆ ⁻ +e→NbCl₆ ²⁻  Eq. 2

NbCl₆ ²⁻ +e→NbCl₆ ³⁻  Eq. 3

The attribution of the two reduction waves to partial reduction of the niobium-containing anionic species was also confirmed by Koichi, who reported that the main niobium anionic species in the 33.3% NbCl₅—66.7% EMIC melt was NbCl₆ ⁻, forming a complex with EMI⁺ cations. Therefore, the two reduction waves at −0.86 V and −1.83 V vs Pt. can be attributed to Nb(V)/Nb(IV) and to Nb(IV)/Nb(III) redox couples, respectively. The reduction wave peak at −0.86 V vs. Pt is very close to the Sn reduction peak recorded in SnCl-EMIC solutions, suggesting the possibility of co-deposition of the two elements. Compared to Sn, more intense current density peaks were measured. Further increasing the cathodic potentials resulted in c.d. values approaching those of pure EMIC, with a shifting of the current increase from −2.0 V in pure EMIC to −2.7 V. Galvanostatic deposition experiments were performed on this solution in constant current mode at c.d. values of 10-30-50-100 mA/cm², by varying the quantity of NbCl₅ in the melt from 10% to 30%. No metallic coating were obtained even after 3600 s.

The CV of 75% EMIC-21% NbCl₅—4% SnCl₂, showed one pronounced reduction peak at −1.83 V and c.d. of 13.4 mA/cm² (FIG. 1-c), probably corresponding to the reduction of Nb(IV) to Nb(III), indicating that the potential for alloy deposition is slightly shifted towards more negative values compared to the reduction potential of either cations from the respective electrolyte. Notably, the peak is preceded by a slowly rising c.d. in the potential range where—according to curve (a)—Sn2+ ions can be reduced to Sn and Nb(V) ions to Nb(IV) ions. A number of galvanostatic tests were performed on EMIC based electrolytes, varying the quantity of NbCl₅ and SnCl₂ salts, and using both niobium and tin anodes. Sn anodes visibly dissolved by simple immersion in the electrolyte and were severely consumed at the end of any electrodeposition process. For example, electrochemical deposition carried out at constant current density of 10 mA/cm² and 130° C. for 10 minutes gave a bright film, having thickness of about 1.2 □m thick and a niobium content of 3 wt %. In the same experimental conditions the niobium anode was not chemically etched by the electrolyte nor did it appear to be damaged after the deposition test. Correspondingly, niobium content in the resulting film increased up to 8 at %, and thickness was 550 μm. Therefore, it was concluded that the Nb sheet should be preferred to Sn as anodic material in the considered process.

FIGS. 2A-2C2-a shows the effect of a galvanostatic polarization on the cathodic potential: at 10 mA/cm² and 30 mA/cm² the potential stabilized immediately after the beginning of the experiment, while a pronounced polarization at negative potential values appeared at 50 mA/cm², the current reaching the plateau more slowly. Niobium content in the coating increased with current density up to 35 at % at 50 mA/cm². Correspondingly the thickness of the coating decreased to 150 nm. In FIGS. 2A-2C2-b it is reported the surface morphology of a Nb—Sn coating obtained from obtained from a 85% EMIC-10% NbCl₅—5% SnCl₂ solution at 120° C. on a copper substrate and at a constant current density of 40 mA/cm² applied for 240 s. The coating appeared to be porous. The maximum niobium content in the sample was 18 at %, and the thickness was 1 μm. FIGS. 2A-2C2-c shows the XRD pattern of Nb—Sn films deposited at 40 mA/cm² and 120° C. for 450 s from the same electrolyte. The Nb content in the deposits was about 35 at %, film thickness was 500 nm. As revealed by the XRD pattern and the powder diffraction references, the Nb—Sn thin film showed a cubic Nb₃Sn structure (A15 phase) with a strong (211) preferred orientation, no NbSn₂ phase was detected.

FIGS. 3A-3C show CV curves over the potential range from −0.4 to −3.5 V corresponding to the pure BMIC (a), BMIC with addition of 10%. SnCl₂ (a), BMIC with addition of 25%. NbCl₅ (b), and BMIC containing 5%.SnCl₂ and 21%.NbCl₅ (c). In the case of pure BMIC, a low current density (c.d.) was observed over the potential range from OCP to −1.9 V, which can be considered a suitable potential window for Nb—Sn deposition. As for EMIC, the significant increase of C.d. towards the lower limit of the scanning range might be attributed to the electrochemical decomposition of the water contained in the electrolyte or to the reduction of ionic liquid itself.

By adding 10%.SnCl₂ to BMIC, a broad reduction wave peaking at about −1.14 V and c.d. of 6.0 mA/cm² was observed. As in the case of EMIC, the melt is basic and SnCl₃ ⁻ is the main anionic specie present in the melt, therefore it reacts according to Eq. 1. Compared to the EMIC-based electrolyte, the reduction of Sn(II) ions shifted to more cathodic potentials and higher c.d, suggesting the possibility of better co-deposition with Nb. Electrodeposition experiments carried out from this solution on copper substrates at 20 mA/cm² and 80° C. for 600 s resulted in a uniform and adherent tin coating (not shown).

By addition of 25% NbCl₅ to BMIC (FIG. 3-b) two pronounced reduction waves were observed peaking at −0.6 V with c.d. of 1.5 mA/cm², and at −1.96 V vs. Pt with c.d. 4.1 mA/cm², corresponding to partial reduction of Nb(V) and Nb(IV) species respectively (see Eq. 2 and Eq. 3). Compared to EMIC electrolytes, the reduction of the couple Nb(V)/Nb(IV) occurs at less cathodic potentials and the reduction of the couple Nb(IV)/Nb(III) to more cathodic potentials, with lower c.d. values. Furthermore, the first reduction peak for Nb(V) appears at potential values less cathodic than the reduction peak of Sn(II). Compared to EMIC electrolytes, the better overlapping of the reduction waves of Sn(II) and Nb(V) can be expected, possibly leading to higher Nb content in the electrodeposited Nb—Sn alloy.

Finally, the cathodic behavior of 75% BMIM-21% NbCl₅—4% SnCl₂ electrolyte was investigated (FIG. 3-c). Two reduction waves peaking at about −1.1 V with c.d. of 1.9 mA/cm2 and −2.0 V with c.d. of 4.5 mA/cm2 were observed. The first reduction wave peaks at potential values very close to that of Sn(II), while the peak owing to the reduction of Nb(V) to Nb(IV) is not evident.

Several deposition tests were carried out from a 85% Bmim-Cl—10% NbCl5—5% SnCl₂ bath. Electrodeposition at 40 mA/cm² for 600 s at 130° C. on a copper cathode and using a niobium anode resulted in a 750 nm film, having a nominal Nb content of 28 wt. %. According to XRD pattern in Error! Reference source not found., the Nb—Sn film includes a cubic Nb₃Sn structure (A15 phase) with strong (211) preferred orientation (reflection at 41.78°), along with a disordered orthorhombic NbSn₂ structure (reflections at 28.64°-29.86 °-57.87°). By increasing the deposition c.d. at 400 mA/cm² for 30 minutes at 130° C., Nb—Sn film containing a maximum of 17 at % Nb and an average Nb content of about 9 at. % was obtained. A significant amount of chloride (up to 6 at %) was also measured. The film thickness was about 200 nm. The sample surface was unevenly porous. The SEM micrograph was not well-defined because of a film of electrolyte covering the surface (Error! Reference source not found.-a and -b). The XRD pattern and the powder diffraction references revealed the presence of a cubic Nb₃Sn phase having (211) preferred orientation. An orthorhombic NbSn₂ phase with a slight (422) preferred orientation was also detected. GD-OES depth profiling analysis was carried out in order to assess the thickness of the Nb—Sn layer and the oxygen content. As shown in Error! Reference source not found.-c, the Nb-Sn layer was about 50 nm thick, followed by a Nb—Sn—Cu and Sn—Cu layers. The presence of a Nb—Sn—Cu layer can be explained considering the thermal interdiffusion occurring at the operating temperature. A very low oxygen content was measured, confirming the good quality of the film. All samples presented the η Cu₆Sn₅ phase. Due to the low signal to noise ratio, it is not possible to estimate the relative amount of the Nb₃Sn and NbSn₂ phases. However, there is evidence of higher NbSn₂ volume percentage in samples obtained at 400 mA/cm².

An alternative approach (in the following also named “approach 2”) was also used, consisting in electrolytic dissolution of the Nb anode during prolonged polarization in a BMIC-SnCl₂ electrolytes. As showed by the SEM micrographs in FIGS. 6-a and -b, samples obtained at 40 mA/cm² for 4 h 30 min were characterized by a rough and uneven surface, with bumps surrounded by a fine grained material. The EDX analysis (not shown) revealed that the bumps mainly consisted in pure tin plus 4 at % Cl and 0.4 at. % Nb. The fine grained phase surrounding the bumps consisted in a Nb—Sn phase containing up to 63at % Nb. a significant Cl content was also measured (up to 11 at %), which can be attributed to precipitation of Nb and Sn chlorides. The average composition was 4 at % Cl, 23 at % Nb and 73 at % Sn. The XRD pattern in FIG. 6-d showed the presence of Cu₆Sn₅, Cu₃Sn and NbSn₂ (2theta=37.63°; 53°; 83°) phases. The reflection at 41.8° can be attributed both at Nb₃Sn and Cu₃Sn phases. GDOES analysis evidenced a Nb and Sn overlapping region about 4 μm thick, and a Sn—Cu layer about 3 μm thick laying underneath, given by Cu—Sn interdiffusion.

Also at 400 mA/cm² the sample surface appeared rough, grainy and inhomogeneous (FIGS. 7-a and -b). EDX analysis (not shown) evidenced that the globular shapes on the surface consisted in pure tin. The average composition was 2 at % Nb, 3 at % Cl and 95 at % Sn. GDOES analysis (FIG. 7-c) revealed a surface Nb—Sn layer about 0.5 μm thick, followed by Sn-rich layer of about 6 μm and by Cu and Sn overlapping region about 8 μm. The XRD pattern (FIG. 7-d) revealed the presence of cubic Nb₃Sn with (211) preferred orientation and of the Cu₆Sn₅ phase. There was no evidence of the orthorhombic NbSn₂ phase. It was concluded that the Nb₃Sn phase was unevenly distributed on the sample surface, between the Sn bumps.

The crystallite size τ was estimated by the Scherrer's equation:

τ=Kλ/β2 cos θ,   Eq. 4

where K is the shape factor (taken as 0.94 for cubic crystals), λ is the X-ray wavelength (1.54 for Cu Kα radiation), β is the line broadening (full width at half maximum, FWHM), and θ is the Bragg angle. The average crystallite size of Nb—Sn coatings electrodeposited from the four baths are reported in Table 2. The crystallite size of Nb—Sn films was affected by the c.d. value rather than by bath composition. Films electrodeposited from either EMIC or BMIC based ionic liquids containing NbCl₅ showed a crystallite size in the range 3÷4 nm at c.d. of 40 mA/cm², and of about 15 nm at c.d. of 400 mA/cm². Films deposited by anodic dissolution of Nb showed higher crystallite size, of about 25 nm at c.d. of 40 mA/cm² and about 62 nm at c.d. of 400 mA/cm². The average surface roughness was about 0.233 μm in the former case, and in the range 0.696÷2.113 μm using approach 2.

TABLE 2 Bath type, deposition current density (c.d.), crystallite size (τ), thickness (t), average roughness (Ra) and root mean square roughness (Rq) of Nb—Sn films. Bath c.d. τ t Ra Rq type (mA/cm²) (nm) (μm) (μm) (μm) 1 40 3 1.000 — — (XRF) 2 40 4 0.750 — — (XRF) 2 400 15 0.200 0.233 ± 0.026 0.306 ± 0.054 (XRF) 3 40 25 1.446 ± 0.308 1.928 ± 0.229 4 40 24 4.000 0.696 ± 0.051 0.886 ± 0.054 (GDOES) 4 400 62 0.500 2.113 ± 0.676 2.934 ± 1.229 (GDOES)

Electrodeposition of Nb—Sn thin films from ionic liquids and without the need of high temperature heat treatment is disclosed herein. Electrolytes consisted of either EMIC or BMIC with addition of SnCl₂ and NbCl₅ salts. Cyclic voltammetry (CV) demonstrated that the selected ionic liquids had a sufficiently wide potential window to allow the electrodeposition of Nb—Sn alloys, notwithstanding a relative high water content in the electrolyte.

The electrodeposited Nb-Sn thin films with average Nb content up to 63 at % showed a cubic Nb₃Sn structure (A15 phase) with (211) preferred orientation. Other phases were also observed, in particular the disordered orthorhombic NbSn₃ phase, the pure Sn phase and Cu₆Sn₅ structure, depending on the operating parameters. Realization of improved intra-crystal structure and inter-grain boundary characteristics in the Nb₃Sn material layer in a controlled environment using pulsed electrodeposition, stringent temperature control of the electrodeposition bath, and optimization of the galvanic cell design for better thickness uniformity and layer microstructure are the chief challenges at this stage.

Overall, the electrodeposition of Nb—Sn from EMIC and BMIC based ionic liquids, even in the presence of a relatively high content of water, was shown to be a promising process for the deposition of Nb₃Sn thin films on copper substrates. As electrochemical deposition is controllable on curved surfaces and is also scalable in size, in principle this technique could allow using superconductors as surface coatings as opposed to bulk, wires and cables.

Synthesis of Superconducting Nb₃Sn Coatings on Nb Substrates

Superconducting Nb₃Sn films are obtained by electrodeposition of Sn layers and Cu intermediate layers onto Nb substrates followed by high temperature diffusion in inert atmosphere. Electrodeposition was performed from aqueous solutions at current densities in the 20 to 50 mA/cm² range and at temperatures between 40 and 50° C. Subsequent thermal treatments were realized to obtain the Nb₃Sn superconductive phase. Glow discharge optical emission spectrometry (GDOES) demonstrated that after thermal treatment interdiffusion of Nb and Sn occurred across a thickness of about 13 μm, where the Nb₃Sn phase was about 5 μm thick. X-ray diffraction (XRD) patterns confirmed the presence of a cubic Nb₃Sn phase (A15 structure) having (200) preferred orientation. Electrical superconductivity tests measured a maximum J_(c) (4.2 K, 12 T) of 600 A/mm² in perpendicular magnetic field. The J_(c) (4.2 K, 12 T) in parallel magnetic field was 736 A/mm². With the procedure described herein, coating complex shapes cost effectively becomes possible, which is typical of electrochemical techniques. Furthermore, this approach can be implemented in classical wire processes such as “Jelly Rod” or “Rod in Tube”, or directly used for producing superconducting surfaces.

Disclosed herein is a combination of thermal diffusion processes and electrochemical techniques to obtain thick superconductive Nb₃Sn coatings onto Nb substrates. In certain embodiments, Nb₃Sn coatings of at least 5 microns can be obtained. The approach was to electrodeposit a seed copper layer onto the Nb substrate, followed by a tin layer and a copper barrier layer. The electrodeposition processes were carried out using aqueous solutions working at near-room temperatures and atmospheric pressure. Samples were then heat treated and characterized. Details of the fabrication process are given in the following.

Experimental Setup

Electrodeposition tests were carried out on niobium foils of 1 cm×3 cm having thickness of 25 μm and of 250 μm. Prior to deposition, the niobium foils were degreased in acetone and cleaned in diluted acid to reduce the presence of niobium oxides on the surface. In fact, niobium oxides could reduce the adhesion of electrodeposited metals and act as a diffusional barrier layer during the heat treatment, hindering the formation of the superconductive phase. The electrodeposition of tin was performed using the commercial bath Solderon™ MHS-W at a current density of 50 mA/cm² and bath temperature of 50° C. Copper seed layers were electrodeposited at 30 mA/cm² and 40° C. using a sulphate-based electrolyte whose composition is reported in Table 3. Copper barrier layers were deposited from a pyrophosphate-based electrolyte whose composition is reported in Table 4. The pH of the electrolyte was 8.5. Electrodeposition experiments were carried out at 20 mA/cm² and 50° C. Electrolytes were prepared from analytical grade chemicals and deionized water. Electrodeposition experiments were made in a two electrodes cell, where the anode was a copper sheet and the cathode a Nb foil. Deposition times ranged from 1 to 25 min.

TABLE 3 Composition of electrolyte used for the deposition of the copper seed layer. Chemicals Concentration (g/l) CuSO₄ 60 H₂SO₄ 200 g/l HCl  40 g/l

TABLE 4 Composition of the electrolyte used for the deposition of the copper barrier layer. Chemicals Concentration (g/l) Cu₂P₂O₇ 26 NaNO₃ 5 Na₄P₂O₇ 180

Heat treatments were performed in a computer controlled tubular furnace. The oven was equipped with three separately programmable induction resistances. Temperature was continuously monitored and maintained constant by means of two thermocouples. Heat treatments were performed in argon atmosphere. To determine the diffusional parameters, samples were observed using an optical microscope. The samples were prepared for optical microscope observation by means of classical metallurgical techniques: they were placed in an epoxy resin and accurately sliced by means of a metallographic sectioning saw. The exposed surface was grinded by means of a Buheler HandiMetr roll grinder, using sandy papers from a 240 grit to 600 grit. The final polishing was performed by an automatic grinding and polishing system (LECO GPX-300). Samples were observed after heat treatment using an inverted metallurgical microscope (Nikon ECLIPSE MA200,) connected to a computer with a camera control unit. The Imaging Software used for the analysis was “NIS-Elements”, which gives the possibility to apply smart filters to the image, such as different phase and grain boundary recognition. The maximum optical magnification was 500×.

The crystallographic structure of the Nb—Sn coatings was assessed by X-ray diffraction (XRD) using a Philips PW1830 instrument, with Cu Kα1 radiation and Bragg-Brentano geometry. XRD was performed in the 2θ angular range of 10 to 90°. An approximate measure of the grain size was evaluated by calculating the crystal coherence extensions according to Scherrer equation. Glow discharge optical emission spectrometry (GDOES) depth profiling analyses were performed with a Spectruma GDA750 analyser using argon ions for sputtering with a beam spot size of 2.5 mm. For superconductivity tests, a commercial magneto-cryostat equipped with a Variable Temperature Insert (VTI) was used, whose operation temperature was in the range of 1.5 to 200 K. Since the Nb—Sn—Cu films after reaction could not be soldered, the original soldered contacts used to transfer the current from the Cu current leads to the samples were replaced by a sample holder with pressure contacts. For the same reason, the voltage tap wires were attached to stainless steel screws that were put in contact with the sample. Using the modified setup, film samples were tested for critical current I_(c) in liquid He at 4.2 K and in magnetic fields from 0 T up to 14 T. The tests were performed both in a field parallel and perpendicular to the tape. An electrical field criterion of E_(c)=1 μV/m was used to define the transition voltage as:

${V_{c} = {E_{c}\rho_{c}\frac{1}{S}}},$

where S is the superconducting tape cross section and 1 is the distance between the voltage taps used during the I_(c) measurement. Thickness and width values were measured with micrometer and caliber respectively in five positions along the samples. The average length of the samples was 37.41±0.51 mm.

Results Electrodeposition of Sn and Cu Layers on Nb Substrates

Preliminary electrodeposition experiments of Sn layers were carried onto Nb substrates at 50 mA/cm² and 50° C. The coating showed high roughness and scarce adhesion on the Nb. On the other hand, the electrodeposition of Sn onto copper substrates resulted in a bright Sn coating with good adhesion. In addition, as shown in FIG. 8, the growth rate of the Sn film on copper substrates was about 1.25 μm/min), higher than that onto Nb substrates (about 0.63 μm/min). The most efficient thermal techniques for obtaining superconductive Nb₃Sn wires through solid diffusion at high temperature require the presence of copper and the formation of a Cu—Sn phase prior to the growing of the Nb₃Sn phase. Due to the presence of copper, a Nb—Sn—Cu ternary system forms. The diffusion path from the Cu—Sn solid solution to the Nb—Sn solid solution passes only through the A15 phase field, destabilizing the formation of the non-superconductive phases NbSn₂ and Nb₆Sn₅. Thus Nb₃Sn is the only phase formed at the interface between Nb and a Cu—Sn solid solution. In short, the addition of Cu lowers the A15 formation temperature from well above 930° C. to any other that is deemed practical thereby limiting grain growth and thus retaining a higher grain boundary density required for flux pinning Although Cu can be detected in the A15 layers, it is generally assumed to exist only at the grain boundaries and not to appear in the A15 grains, allowing to use the binary A15 phase diagram to qualitatively interpret compositional analysis in wires. Also, to the first order, the addition of Cu does not dramatically change the superconducting behavior of wires as compared to binary systems. Therefore, the conclusion was made that a copper seed layer could be conveniently deposited onto the Nb substrates prior to the deposition of the Sn film.

Electrodeposition of copper on Nb was carried out from a sulphate-based electrolyte at 30 mA/cm² and 40° C. The resulting coating was bright and adherent (FIG. 9-a). As expected, the subsequent deposition of tin on the copper seed layer resulted in a coating adherent to the substrate (FIG. 9-b). The XRD pattern revealed that the Sn coating onto the copper substrate had a crystalline tetragonal structure with a slight (211) preferred orientation.

Thermal treatments on Nb/Cu/Sn samples (see the following section) evidenced that Sn coalesces into small lumps during heating, producing a severe inhomogeneous tin distribution. This issue was addressed by changing sample design. A copper barrier layer was deposited onto the Sn coating in order to restrain the coalescence effect and maintain the coverage of the Nb substrate homogeneous. The copper barrier layer was deposited from a pyrophosphate-based electrolyte at 20 mA/cm² and 50° C. Thermal treatments for superconductivity tests were carried out on samples having the sandwiched structure represented in FIG. 9-d. As reported in Table 7, three types of samples were fabricated, based on the thickness of the tin and copper barrier layers. The thickness of the Sn layer ranged between 10 and 20 μm, and the thickness of the copper barrier layer was either 10 μm or 15 μm.

Calculation of Diffusional Parameters

The diffusional parameters were determined by annealing Nb/Cu/Sn samples. In the case of Nb—Sn systems, a parabolic growth rate was suggested in literature for the newly forming superconductive layer. This behavior is derived from first Fick's law, assuming a constant concentration of the diffusing component at both the boundaries of the interlayer, and a constant concentration gradient across the interlayer:

$\begin{matrix} {{J_{i} = {{- D_{i}}\frac{\partial C_{i}}{\partial x}}},} & {{Eq}.\mspace{14mu} 5} \end{matrix}$

where J is the diffusional flux (mol/μm²·s), D is the diffusion coefficient (μm²/s), C is the concentration (mol/μm³) and x is the width of the concentration gradient (μm). More precisely the parabolic growth can be described by the simple law:

$\begin{matrix} {{L = \sqrt[n]{2{Dt}}},} & {{Eq}.\mspace{14mu} 6} \end{matrix}$

where L is the thickness of the new phase created after the heat treatment (μm), n is usually considered equal to 2, D (μm²/s) is the interdiffusion coefficient and t is the duration of the heat treatment in seconds. Large deviations from the parabolic growth rate are primarily due to cracks in the layers for n<2, and to depletion of Sn in the matrix for n>2. The interdiffusion coefficient can be written in an Arrhenius form as:

$\begin{matrix} {{D = {D_{0}{\exp \left( {- \frac{Q_{0}}{RT}} \right)}}},} & {{Eq}.\mspace{14mu} 7} \end{matrix}$

where D₀ (μm²/s) is the diffusion frequency, Q₀ (kJ/mole) is the activation energy for diffusion, R is the gas constant (kJ/K·mol) and T (K) is the reaction temperature. The thickness of the newly formed phase was sampled in ten different locations and the average value was calculated (Table 5).

TABLE 5 Temperature and duration of thermal treatments, thickness of the resulting Nb—Sn phase and corresponding standard deviation. Temperature Duration Thickness of Nb—Sn Sample (° C.) (h) phase (μm) 1 220 20 No diffusion 2 400 20 No diffusion 3 800 40 4.2 ± 0.36 4 800 20 3.9 ± 0.44 5 950 20 12.7 ± 0.48 

At temperatures below the melting point of tin (232° C.), diffusion was negligible. At higher temperatures, diffusion becomes significant, but the tin layer coalesced forming small domains and leaving the niobium substrate partly uncovered. The thickness of the newly formed Nb—Sn phase could be evaluated in the areas where good coverage was maintained. However, in view of further analysis and practical applications, the samples design was later changed (see following section).

In FIG. 10, the experimental thickness of the Nb—Sn phase is reported as a function of the duration of the heat treatment, and it is compared to curves calculated based on data reported in literature. In Table 6 the calculated values for D₀ and Q₀ are reported and compared with others found in literature. The experimental activation energy for diffusion Q₀ was about 202 kJ/mol. Values reported in literature are between 221 kJ/mol and 404 kJ/mol. High values of Q₀ are not desired because it means lower diffusion rates and higher duration of heat treatments, with excessive grain growth and loss of the superconductive properties. As shown in Table 6, the value of Q₀ determined in the present work is lower than that measured for Nb surrounded by a bronze matrix.

TABLE 6 Experimental D₀ and Q₀, and values of Q₀ reported in literature. Sample design before TT D₀ (μm²/s) Q₀ (kJ/mol) Sn/Nb — 221 CuSn7/Nb — 404 CuSn8/Nb — 279 This work 2 × 10⁹ 202

Heat Treatment of Nb/Cu/Sn/Cu Samples

As mentioned before, it was observed that during thermal treatments of Nb/Cu/Sn samples tin melts at temperatures higher than 232° C. and coalesces on the Nb surface. To overcome this problem a Cu barrier layer was electrodeposited onto the Sn coating. Three different types of samples were produced, whose design is shown in FIG. 9-d and the thickness of each layer is reported in Table 7. The thickness of the Sn layer ranged between 10 and 20 μm, the thickness of the copper barrier layer was either 10 μm or 15 μm.

Nb/Cu/Sn/Cu samples were processed following the thermal profile shown in FIG. 11. The initial step was carried out at a temperature of 214±2° C. for 72 hours, slightly lower than the Sn melting point, to allow for relaxation of the internal stresses in the metal layers and to start the diffusion between Cu and Sn. According to literature, during this step a 3 μm η phase should form. The second intermediate step at 458±2° C. for 10 hours was done to allow the formation of a liquid tin phase and start the interdiffusion with niobium and copper. This intermediate step was necessary in order to avoid the Kirkendall effect and consequently a severe degradation of the materials properties. Furthermore, higher temperatures would induce higher Sn pressures onto the Cu barrier layer, with a possible damage of the same and consequent Sn leakage. According to literature, after ten hours at ˜450° C., a bronze E phase forms on the surface, and an η phase develops underneath. Finally, the temperature was increased to 700±1° C. for a duration of 24 hours to form the Nb₃Sn superconducting phase with a mean grain size of about 80 μm. After the heat treatment, small tin islands were observed only on the surface of type 1 samples, probably because of the lower thickness of the copper barrier layer.

TABLE 7 Thickness of Sn and Cu layers in Nb/Cu/Sn/Cu samples. Sample Thickness of Sn layer (μm) Thickness of Cu layer (μm) Type 1 10 10 Type 2 15 15 Type 3 20 15

Characterization of Nb/Cu/Sn/Cu Samples

Nb/Cu/Sn/Cu samples after thermal treatment were characterized by means of GDOES, XRD and electrical tests. By means of GDOES analysis, the region where Sn and Nb are superimposed was defined and, in some cases, the possible position and thickness of the Nb₃Sn phase was inferred. It must be noted that the in the present work the relative intensities of the GDOES signal do not give indication on the relative amount of the elements.

According to the experimental diffusional parameters, the expected thickness of the Nb—Sn alloy after thermal treatment was about 3.5 μm. In FIG. 12-a the qualitative composition profile of a type 1 sample consisting of a Nb/Cu/Sn(10 μm)/Cu(10 μm) multilayered structure is shown. The thickness of the Nb₃Sn phase was about 5 μm, located at a depth of about 10 μm from the surface. The corresponding XRD pattern (FIG. 12-b) revealed the presence of a crystalline cubic Nb₃Sn phase (A15 structure). Other phases were also detected: NbSn₂, β Sn, ε Cu₃Sn (2θ=43.65°), NbO and NbO₂ (36.98° and 74.84°).

In FIG. 13-a the GDOES analysis of a type 2 sample consisting of a Nb/Cu/Sn(15 μm)/Cu(15 μm) multilayered structure after thermal treatment is shown. The composition gradient did not allow to infer the thickness nor the position of the Nb₃Sn phase. However, the XRD pattern (FIG. 13-b) shows the reflection of a crystalline cubic Nb₃Sn phase (A15 structure). Other reflections can be attributed to NbSn₂, β Sn, Cu—Sn phases, NbO and NbO₂.

In FIG. 14 the GDOES analysis and corresponding XRD pattern of type 3 sample (Nb/Cu/Sn(20 μm)/Cu(15 μm)) after thermal treatment are reported. Based on the GDOES analysis, the thickness of the Nb₃Sn phase may be about 5 μm. According to the XRD pattern, a Nb₃Sn phase having cubic structure with a strong (210) preferred orientation is present. Compared to the other XRD patterns, the signal to noise ratio increased, probably due to mechanical cleaning of the sample surface before XRD analysis. NbSn₂ (2η=18.71° and 58.19°), NbO₂ (26.14°-35.28°-52.13°, Cu₆Sn₅) (68.27°), β Sn (30.24°), Cu (30.24°) were also detected. In all cases, the GDOES analysis revealed that the region where Sn and Nb elements are superimposed was about 13 μm thick and confirmed the presence of oxygen in the outer 3 μm.

Since the dominant source of flux pinning in Nb₃Sn appears to be grain boundaries, in order to obtain high critical current densities it is necessary to produce a fine grained structure. The crystallite size τ of samples after thermal treatment was estimated by the Scherrer's equation:

τ=Kλ/β2 cos θ,   Eq. 8

where K is the shape factor (taken as 0.94 for cubic crystals), λ is the X-ray wavelength (1.54 for Cu Kα radiation), β is the line broadening (full width at half maximum, FWHM), and θ is the Bragg angle. The average crystallite size of the electrodeposited Nb—Sn alloys was about 27 nm for type 1 sample, 24 nm for type 2 and 32 nm for type 3 sample. In similar conditions Verhoeven obtained grain size in the range 100-110 nm.

The electrical tests were performed both in a field parallel and perpendicular to the tape. To calculate the critical current density J_(c) from the measured currents I_(c), a thickness of 5 μm was assumed for the Nb₃Sn phase in all samples. FIG. 15 shows the critical current density as a function of the magnetic field in the perpendicular (a) and parallel (b) orientations. A superconductive behavior was observed. The largest J_(c) (4.2 K, 12 T) in perpendicular magnetic field of 600 A/mm² was obtained for a type 2 sample. The corresponding J_(c) (4.2 K, 12 T) in parallel magnetic field was 736 A/mm². This is consistent with crystallite size values for unoriented samples (Table 8). Despite higher crystallite size, type 3 sample showed intermediate electrical properties, which might be explained considering that these samples have a strong (210) preferred orientation, which is an additional parameter affecting electrical properties.

TABLE 8 Preferred orientation (P.O.), crystallite size, critical current and critical current density of Nb₃Sn thin films. Cristallite size I_(c) J_(c) (4.2 K, 12 T) J_(c) (4.2 K, 12 T) Sample (nm) (A) (A/mm²) perp. (A/mm²) par. Type 1 27   6.5 148 — Type 2 24 24 600 736 Type 3 31  16* 420 —

The results of the synthesis of Nb₃Sn thin films onto Nb substrates were presented. Superconductive coatings were obtained by combining thermal treatments and the electrochemical technique for thin film deposition. Samples were fabricated by electrodeposition of a Cu seed layer onto the Nb substrate, followed by deposition of a Sn layer (10-20 μm) and a Cu barrier layer (10-15 μm). Subsequent thermal treatments were carried out to form the Nb₃Sn phase. The copper seed layer improved adhesion of tin onto the substrate, while the copper barrier layer limited tin coalescence during thermal treatments. Both layers were expected to favor the formation of the Nb₃Sn phase.

Diffusional parameters were determined, indicating a thickness of the Nb—Sn phase after thermal treatment of about 3.5 μm. GDOES analysis revealed that the region where Sn and Nb are superimposed was about 13 μm thick. In some cases it was possible to infer that the thickness of the Nb₃Sn phase was about 5 μm, at about 10 μm from the sample surface. The XRD patterns revealed the presence of both Nb₃Sn+NbSn₂ crystalline phases and of Cu—Sn phases. Electrical tests showed superconductive behavior. The largest J_(c) (4.2 K, 12 T) in perpendicular magnetic field was 600 A/mm² and the J_(c) (4.2 K, 12 T) in parallel magnetic field was 736 A/mm². 

What is claimed is:
 1. A method comprising: electrodepositing a film comprising a Nb—Sn material onto a copper substrate surface from an electrolyte bath comprising (a) SnCl₂, (b) NbCl₅, and (c) (i) 1-Ethyl-3-methylimidazolium chloride (EMIC), (ii) 1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture thereof.
 2. The method of claim 1, wherein the bath comprises (a) SnCl₂, (b) NbCl₅, and (c) 1-Butyl-3-methylimidazolium chloride (BMIC).
 3. The method of claim 1, wherein prior to initiation of electrodeposition (a) is present in an amount of 1 mol % to 50 mol %, (b) is present in an amount of 1 mol % to 50 mol %, and (c) is present in an amount of 1 to 99 mol %.
 4. The method of claim 1, wherein the film comprises Nb₃Sn.
 5. The method of claim 1, wherein the electrodeposition occurs at 1 to 1000 mA/cm² and 0 to 150° C. for 1 to 7200 seconds.
 6. The method of claim 1, wherein the Nb—Sn film has a structure of cubic Nb₃Sn, orthorhombic NbSn₂, η Cu₆Sn₅ and ε Cu₃Sn.
 7. A method comprising: electrodepositing a seed copper layer onto a surface of a Nb substrate; electrodepositing a tin layer onto the seed copper layer; electrodepositing a copper barrier layer onto the tin layer to form an intermediate construct; and heating the intermediate construct to form a Nb₃Sn coating.
 8. The method of claim 7, wherein the heating of the intermediate construct is from 10 to 90° C.
 9. The method of claim 7, wherein the Nb₃Sn coating has a structure of cubic Nb₃Sn, orthorhombic NbSn₂, η Cu₆Sn₅ and ε Cu₃Sn. 